Just bought a 2005 gmc 2500hd from a used car lot. When we try to shift into four wheel drive it will not engage, nor will the light come on saying that it is on. We have the shifter on the floor, not push-button. where do you first start to look for the problem?

There are a few potential issues that could be causing your 2004 GMC Envoy SLT to go into 4-low but not 4-high or 2-wheel drive. Some possible causes include:

1. Transfer case issues: The transfer case is responsible for controlling the engagement of the different drive modes. If the transfer case is damaged or malfunctioning, it may not be able to engage 4-high or 2-wheel drive.
2. Shift motor issues: The shift motor is responsible for controlling the engagement of the transfer case. If the shift motor is damaged or malfunctioning, it may not be able to engage 4-high or 2-wheel drive.
3. Vacuum issues: The vacuum system is responsible for controlling the engagement of the transfer case. If there is a problem with the vacuum system, it may not be able to engage 4-high or 2-wheel drive.
4. Wiring issues: The wiring that controls the transfer case and shift motor can become damaged or corroded over time. This can cause the transfer case to not engage 4-high or 2-wheel drive.
5. Control module issues: The control module that controls the transfer case and shift motor can fail. This can cause the transfer case to not engage 4-high or 2-wheel drive.

It's best to diagnose the issue by a professional mechanic, who can perform a proper diagnosis and repair the issue. They will be able to determine the specific cause of the problem and recommend the appropriate repairs.

The reason the light is flashing constantly is because the vehicle is sensing that four wheel drive(4WD) is not working. These 4WD systems use vacuum from the engine to engage/disengage to front axle. The vehicle probably has no vacuum going to the front axle, which would not allow the 4WD shift to complete, which would then leave the light on the dash flashing.
First thing to check is that the vacuum actuator on the firewall is working as it should.

Try shifting the shift selector into Neutral first then try engaging 4wd low. If that doesn't work try shifting into 1st gear and then try engaging 4wd low. The information is inside your owners manual but if this doesn't work please let me know. Also is the light on for which mode your truck is in (2wd or 4wd high)? If not, you may want to check for a blown fuse.

Not good! there is a sensor in your transfer case that senses that you are now in four wheel drive if your transfer case does not lock in it will set a code and the light will flash and default back to two wheel drive. check the fluid level in the transfer case and front differential for a sign of internal failure. that is causing the grinding sound.

just shift to 4x4 on the fly as long as you are not going faster than 80 kms an hour.
four wheel drive low gear you will need to stop and put it in neutral, keep the brake to the floor and shift.

This is normal if you are on black top or concrete.The 4wd is only meant for mud or snow not dry pavement.Hope this helps.Good luck.

the auto 4 wheel does the same as part time in jeeps, it engages only when the vehicle loses traction. when auto 4 wheel is on the truck isn't fully in 4 wheel drive. my 02 sierra makes the clunk noise too but it does engage. try checking your fluids, i know its way pass the time for me to change mine out.

check the shift solenoid on the tranfer case also check the voltage going to it! make sure theres 12 volts and check the ground also check the fluid level in the transfer case

try reversing six to eight feet after shifting into 4WD. PLEASE RATE MY ANSWER.

Check fluid level in transfer case.